1. Field of the Invention
The invention generally relates to the field of microlithography, and in particular to objectives used in projection exposure apparatus or mask inspection apparatus. The invention is particularly concerned with correcting, or more generally varying, a light irradiance distribution in a projection light path in such objectives.
2. Description of Related Art
Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV), vacuum ultraviolet (VUV) or extreme ultraviolet (EUV) light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens. A typical value for the magnification is β=±¼.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning mechanism. Commonly used masks contain opaque, transparent or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example.
One of the essential aims in the development of projection exposure apparatus is to be able to lithographically produce structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus. Furthermore, the more devices can be produced on a single wafer, the higher is the throughput of the production process.
The size of the structures that can be generated depends primarily on the resolution of the projection objective being used. Since the resolution of projection objectives is inversely proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep or vacuum ultraviolet spectral range. Also apparatus using EUV light having a wavelength of about 13 nm are meanwhile commercially available. Future apparatus will probably use EUV light having a wavelength as low as 6.9 nm.
The correction of aberrations (i.e. image errors) is becoming increasingly important for projection objectives having a very high resolution. Other important issues are undesired irradiance variations in field and pupil planes of the objective.
An undesired irradiance variation in the image plane directly translates into CD variations, i.e. variations of the critical dimensions. Irradiance variations in the pupil plane are more difficult to understand. The amplitude part of the complex pupil transmission function describes the angular transmission properties of the objective, while the phase part of the pupil transmission function defines its aberrations.
Mathematically, imaging can be described by two Fourier transforms, namely one from the object plane to the pupil plane and one from the pupil plane to the image plane. Prior to the second Fourier transform, the complex pupil transmission distribution must be multiplied by the optical transfer function (OTF) of the imaging system. The OTF can be split into a phase term W describing the aberrations and an amplitude term A describing how the angular irradiance distribution is affected by the objective. Both terms are generally functions of the pupil coordinates (i.e. of ray directions at field level) and of the field coordinates. This expresses that the amplitude of a light ray generally depends on the position where the light ray impinges in the field, and also on the direction of the light ray. Similar considerations apply also to the phase.
If the term A describing the amplitude distribution has an odd symmetry in the pupil coordinates, this will result in a non-telecentric objective. The term telecentricity denotes the mean direction of a light bundle emerging from or converging to a point in a field plane. In a non-telecentric objective overlay becomes a function of focus, because more light reaches a given point on the image plane from one side than from the other, with the result that if the wafer is moved up or down with respect to the image plane (thus defocusing the exposed image), the image effectively moves horizontally.
If the term A describing the amplitude distribution has an even symmetry so that the light irradiance reaching a given point on the image plane is symmetrical, this affects the optimal exposure dose as a function of structure density (pitch) and orientation: lines of different pitch require a different exposure dose to be printed at the same size.
Apodization is used for eliminating adverse effects that are associated with undesired variations of the irradiance distribution in the pupil plane. The term apodization as used herein generally denotes a modification of the amplitude term A of the OTF by using a filter. Sometimes the term apodization is used in the art to denote an optical filtering of the transmittance in a pupil plane so as to suppress the energy of diffraction rings in an objective.
Usually, there is an ideal irradiance distribution in the pupil plane, and the apodization filter is used to correct the real irradiance distribution so that it approaches at least to some extent the ideal irradiance distribution. Sometimes, however, no correction in this sense is required. For example, it may be possible to vary the irradiance distribution in such a manner that adverse effects caused by the non-ideal irradiance distribution apodization can be eliminated by other measures. Such measures include, among others, modifications of the angular light distribution produced by the illumination system, or displacements of lenses contained in the objective or of the wafer.
If the real irradiance distribution does not vary, it usually suffices to use an apodization filter having a fixed spatial filter function, i.e. an attenuation distribution that cannot be modified. In microlithographic projection exposure apparatus, however, the real irradiance distribution often varies at least to some extent so that it is desirable to be able to vary the filter function of the apodization filter.
U.S. Pat. No. 5,444,336 discloses a projection objective of a microlithographic projection exposure apparatus in which different grey filters can be inserted into the pupil plane of the objective. However, the number of different filter functions is necessarily restricted.
US 2006/0092396 discloses a projection objective of a microlithographic projection exposure apparatus in which a variable apodization filter formed by an array of individually programmable elements, for example LCD cells, is arranged in a pupil plane of the objective. By controlling the elements of the array individually, the attenuation distribution of the apodization filter can be varied. One drawback of this known approach is that it is difficult to finely adjust the attenuation produced by each element.
US 2010/0134891 A1 discloses another variable apodization filter for an objective of a microlithographic projection exposure apparatus. Here a reflective coating applied on a curved mirror surface is detuned so as to locally vary the coefficient of reflection of the mirror. A similar approach is also described in U.S. Pat. No. 7,791,711 B2. However, with this approach it is difficult to ensure that the detuning can be completely reversed.
WO 2011/116792 A1 discloses a wavefront correction device in which a plurality of fluid flows emerging from outlet apertures enter a space through which projection light propagates during operation of the projection exposure apparatus. A temperature control unit sets the temperature of the fluid flows individually for each fluid flow. Since the refractive index of a fluid depends on its temperature, this makes it possible to produce a three-dimensional refractive index distribution. The latter is determined such that optical path length differences associated with the refractive index distribution correct wavefront deformations.